The region of flow close to an object is known as the boundary layer. Scientists and engineers have long sought methods of determining quantitative empirical data relating to fluid flow within the boundary layer. Scientists and engineers have used several techniques, including digital particle image velocimetery (DPIV) and digital particle tracking velocimetry (DPTV) to learn about the characteristics of fluid flows, in general. These methods, however, fail to provide robust data for fluid flow in a boundary layer or other shear flows near interfaces.
FIG. 1 is a diagram of a boundary layer profile 100. When a body moves relative to a surrounding fluid, a boundary layer exists very close to the body surface 102 as a result of the ‘no-slip condition’ and viscosity. Consider an object held stationary in a uniform oncoming flow with velocity U. The fluid in direct contact with the body surface 102 adheres to the surface 102 and has zero velocity. The fluid just above the surface 102 is slowed by frictional forces associated with the viscosity of the fluid. The closer the fluid is to the surface 102, the more it is slowed. The result is a thin layer where the tangential velocity, u, of the fluid increases from zero at the body surface 102 to a velocity close to U. This velocity at the outer edge of the boundary layer, “the exterior velocity,” Ue, depends on the shape of the body. By definition, the boundary layer extends from the object's surface 102, y=0, to a position y=d, where the tangential velocity relative to the object's surface 102 is 0.99 Ue (“d99”), or 0.95 Ue (“d95”). The curve representing the continuous variation in tangential velocity from y=0 to y=d is commonly referred to as the boundary layer profile.
Normal velocity relative to the surface also varies from zero at the body surface 102 to some external value, Ve, generating what is known as the v-profile. A third profile, the w-profile, usually exists in the flow over three-dimensional surfaces, where w is tangential to the surface 102 and perpendicular to u.
The shapes of the boundary layer profiles above a particular position on a surface 102 depend on the shape of the body, surface 102 roughness, the upstream history of the boundary layer, the surrounding flow field and the Reynolds number. Flow in the boundary layer can be laminar or turbulent, resulting in radically different classes of profile shapes. The behavior of a body moving relative to a real fluid cannot be accurately described without an understanding of the boundary layer.